The present invention relates to a testing device for materials, and more particularly, to an apparatus which places tensile forces on a sheet, a film, or the like in at least two axes and generally in a radial direction. The testing apparatus can advantageously be employed to test crack propagation and flex fatigue in polymers, elastomers, and rubber.
Throughout recent history numerous devices have been developed and constructed to determine or monitor various mechanical properties of almost any material. One such device is a tensometer, which elongates, often repeatedly, a test material in a single axis. The strength, modulus, elongation as well as other characteristics of materials can be obtained from a tensometer.
U.S. Pat. No. 4,535,636 to Blackburn et al. relates to a mechanical extensometer for use with a constant load creep test machine, wherein the dead weight of the extensometer is counterbalanced by two pairs of weights and connected through a pulley system to a rod extension leading into the furnace where a test sample is undergoing elevated temperature tensile testing. Gripper surfaces, a conical tip and a flat surface, are provided in each sample to reduce the grip pressure normally required for attachment of the extensometer to the specimen and reduce initial specimen bending normally associated with foil-gage metal testing.
U.S. Pat. No. 4,248,096 to Marcum relates to a machine for cyclically applying a force to material specimens for testing the fatigue properties thereof. The specimens are positioned between two members, at least one of which moves angularly in a wobble type of action in a cyclic operation. Enclosure members enclose any suitable desired fluid within which the test specimens are tested.
U.S. Pat. No. 4,574,642 to Fleischman relates to an apparatus for monitoring and measuring the growth of a crack in an elastic specimen. The specimen is repeatedly flexed by a piston at a set frequency and periodically slowed to a substantially lower frequency during which time a line scan camera monitors the length of the crack. The camera is positioned such that the specimen is drawn through the scan line thereof. The camera is calibrated such that the output signal from the camera correlates directly to the cracklength. This output signal is digitized, received, and stored by a digital processor for use in determining the crack growth rate.
ASTM test D4482-85 titled xe2x80x9cRubber Propertyxe2x80x94Extension Cycling Fatiguexe2x80x9d describes a test method that uses a machine commonly referred to as the Monsanto xe2x80x9cFatigue to Failurexe2x80x9d machine. This machine uses a cam to exert a pre-specified extension onto a rubber specimen at a specific cyclic rate. Once again, the sample undergoes a single axis extension.
ASTM test D813-95, titled xe2x80x9cRubber Deteriorationxe2x80x94Crack Growthxe2x80x9d describes a test method that uses a machine commonly referred to as a DeMattia Crack Growth tester. A cured sample with a groove at a 90xc2x0 angle to the specimen length, is cyclically stretched and then compressed, forcing the sample to bend at the groove. The bending motion in the groove stretches the specimen in the low axis direction of the specimen. Again, this exerts a uniaxial stretch in the sample. A small crack or cut can be made in the specimen and the growth of that cut during the test duration is measured.
However, it has been found that the known material testing devices are deficient in being able to simulate natural conditions that polymers, elastomers, and rubber are subject to during actual use. A specific example is a tire sidewall that will experience radial and circumferential strains during its use. Predictions of crack growth in a tire sidewall based on uniaxial testing can be misleading and in some cases cause reversals in the actual performance of different elastomer formulations. Uniaxial and biaxial testing results in different performance levels, see B. J. Roberts and J. B. Benzies, The Relationship Between Uniaxial and Equibiaxial Fatigue in Gum and Carbon Black Filled Vulcanizates, Rubbercon 77, with the biaxial testing being more representative of the actual product performance. The material testing apparatus of the present invention solves the aforementioned problems.
The present invention relates to a material testing apparatus, which creates a straining or stressing force on a sample in at least two axes and generally in a radial direction. The apparatus of the present invention is able to test numerous materials or samples such as sheets and films of various polymeric compositions, such as rubber tire formulations. The apparatus is also suitable for testing and determining crack growth propagation and flex fatigue of samples. By stressing a sample in at least two axes, an accurate measurement is obtained with respect to a radial direction which substantially mirrors results from those obtained from the ordinary, actual, or intended use of the polymer or rubber.
Other advantages and features of the invention will be apparent from the following description of a preferred embodiment and from the claims.